Greases which permit easier handling as compared with lubricating oils are widely used for lubricating various lubrication sites of automobiles and various industrial machines.
There are a number of kinds of greases. For example, JIS (Table 1 of JIS K2220) refers to greases for use in various applications and specifies properties and performance required in respective applications. For example, “grease class 3 for ball or roller bearing” is defined as being applicable over a wide-temperature range, excellent in low-temperature performance and in heat resistance and usable for ball or roller bearings in a temperature range of −30 to 130° C.
In recent years, mechanical parts of automobiles and various other industrial machines have been designed to be operable in a wider temperature range and under more severe lubrication conditions than before. Additionally, as a result of development of new types of machines and mechanical parts, not only operability in a wider temperature range and under more severe lubrication conditions but also performance specific to such machines is now often required.
For example, in recent years, for the transmission of a driving force in a specific direction only, a rotational transmission device with a built-in one-way clutch has been used in automobile auxiliaries such as an alternator, auxiliary driving device and engine crankshaft, for example. The rotational transmission device with a built-in one-way clutch is a device which includes an inner-diameter-side member; a cylindrical, outer-diameter-side member concentrically located around the inner-diameter-side member; ball or roller bearings located between the outer surface of the inner-diameter-side member and the inner surface of the outer-diameter-side member for supporting the inner-diameter-side member and the outer-diameter-side member while permitting relative rotation therebetween; and a one-way clutch adapted for transmitting only such a rotational force that rotates one of the outer-diameter-side member and the inner-diameter-side member relative to the other in a specified direction.
Such an alternator and the like now progress in performance and output and are also used in a wide area including cold climate areas. As a consequence, the conditions under which the rotational transmission device with a built-in one-way clutch is used become severe. Namely, the rotational transmission device is required to operate at a higher revolution speed and a higher load and to achieve a desired performance under an extremely low temperature so as to withstand use in cold climate areas. In this circumstance, a grease used in such a rotational transmission device with a built-in one-way clutch operated under severe conditions is desired to produce a high performance and to satisfy the following characteristics:    (i) The grease must provide satisfactory clutch engagement property (intermeshing ability) at low temperatures. When an engine is started in an extremely cold area in winter, satisfactory clutch engagement property (intermeshing ability) is demanded in order to achieve smooth operation.    (ii) The grease must have excellent performance at high temperatures and provide a prolonged bearing life at high temperatures. As a consequence of severe engine operation conditions, the temperature of location near the engine becomes high. Additionally, automobile auxiliaries are operated at high temperatures for a long period of time. Therefore, the grease must provide a prolonged bearing life at high temperatures.    (iii) The grease must be less apt to cause oil separation under high centrifugal force (acceleration). Since automobile auxiliaries such as alternator, are operated at high revolution speed and used under high centrifugal force, the grease must be less apt to cause oil separation.
It is known that the grease performance at low temperatures may be improved by using a low viscosity base oil. A grease using a low viscosity base oil, however, cannot achieve a good performance at high temperatures, because the base oil is apt to vaporize and to cause oil separation. When, on the other hand, a high viscosity base oil is used, the grease performance at low temperatures is deteriorated though the grease performance at high temperatures is improved.
Namely, the good clutch engagement property as described in (i) above and the long life of bearings in a test at high temperatures as described in (ii) above are generally opposing properties. Thus, when one of the two properties is improved, the other property is deteriorated. It is, therefore, difficult to improve both properties at the same time. Also, to reduce oil separation under a high centrifugal force as described in (iii) above and to improve performance at low temperatures as described in (i) above are also opposing properties.
As conventional greases for use in such a rotational transmission device with a built-in one-way clutch, there are disclosed a grease in which an ether-based base oil such as an alkyl diphenyl ether is used (see, for example, Patent Documents 1 and 2), a grease in which a polyol ester having a kinematic viscosity at 40° C. of 20 mm2/s or less is used (see, for example, Patent Document 3), a grease in which a thickener composed of a diurea compound and a mineral oil, a poly-α-olefin oil or a polyol ester oil is used (see, for example, Patent Document 4), and a grease in which a urea thickener is compounded into an ester-based or synthetic oil-based base oil having a pressure viscosity coefficient of 12 Pa−1 or more (see, for example, Patent Document 5).
The grease using an alkyl diphenyl ether is not satisfactory with respect to low temperature properties, i.e. clutch engagement property at low temperatures. The grease using a base oil containing a polyol ester is unsatisfactory with respect to high temperature property, i.e. the results of a bearing life test at high temperatures are unsatisfactory. Thus, the above two greases cannot satisfy the low temperature performance and high temperature performance at the same time. The other greases using a mineral oil or a poly-α-olefin oil have similar problems. Accordingly, there is a room for further improving the grease.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2006-162032    [Patent Document 2] Japanese Unexamined Patent Application Publication No. H11-82688    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2006-161827    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2006-132619    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2000-234638